The present invention relates to roller cone drill bits having cutter elements that are adapted to reduce the growth of ridges between adjacent kerfs on the borehole bottom. More particularly, the present invention comprises the inclusion of at least one ridge-cutting cutter element adjacent at least one primary cutting element, with the ridge-cutting element preferably having a reduced height and being inclined with respect to the axis of the primary cutting element with which it is associated.
Roller cone drill bits create an uncut region on the bore hole bottom known in the art as xe2x80x9cuncut bottom.xe2x80x9d This is the region on the bore hole bottom that is not contacted by the primary row cutter elements. Primary row cutter elements are the cutting elements that project the furthest from the cone body for cutting the bore hole bottom. If this uncut area is allowed to build up, it forms ridges. As used herein, the term xe2x80x9cridgexe2x80x9d means the uncut formation material that remains between the kerfs cut by adjacent rows of cutter elements as the bit is rotated in the borehole. In some drilling applications, ridges are not significant, because the formation that would form the ridges is easily fractured and ridges do not tend to build up. By contrast, in rock formations that are not easily fractured, or when the formation becomes plastic under the high down hole pressure, ridges tend to build up. The formation of ridges is detrimental to the drill bit, as it causes wear on the cone body and cutter elements, and slows the drill bit rate of penetration.
The increasing use of down hole motors with bent housings and/or bent subs in the drill string assembly for directional drilling introduces a wear characteristic where the outer surface of individual cutter elements becomes heavily worn, while the inner surface reflects relatively little wear. As used herein, xe2x80x9couter surfacexe2x80x9d refers to the side or edge of the cutter element that is closest to gage when the cutter element is at its closest approach to the side wall. Correspondingly, as used herein xe2x80x9cinner surfacexe2x80x9d refers to the side of the cutter element that is closest to the bit centerline when the cutter element is at its closest approach to the side wall. This wear characteristic is particularly caused by the drilling application wherein the drill string is rotated and a bend is employed in the motor housing, which typically can have an angle from 1 to 3 degrees. This causes the circumference of the borehole to increase and causes the ridges that are formed on the borehole bottom to be circumferentially longer than those formed by a bit used without a bent motor housing attached to the drill string assembly. If the ridges are not fractured, the outer surface of the cutter elements encounters increased lateral loads. This leads to excessive wear on both the cutter elements and the cone body. This excessive wear will ultimately lead to breakage or loss of the cutter elements.
Furthermore, the flow of high pressure, abrasive fluid (drilling mud) out of and across the face of the bit causes high rates of bit erosion, particularly in areas where fluid flow is relatively rapid. Channeling of the fluid between cutter elements and recirculation of the fluid around the cutter elements can result in localized rapid fluid flow and undesirable localized erosion.
Hence, it is desired to provide a drill bit that ensures the fracture of the ridges and thereby decreases the wear on the outer surfaces of the cutter elements and on the cone body. It is further desired to provide a bit that mitigates the erosive effect of channelized fluid flow on the bit.
The present invention provides a means to cut the ridges that otherwise may be formed in the uncut area of the bore hole bottom, and a means to provide support to the outer surface of the primary cutter elements which encounter increased lateral loads when the drill bit is used with a down hole motor.
According to the invention, ridge-cutting cutter elements are secured to the cone cutter body and positioned near the primary cutter elements. The ridge-cutting cutter elements may be hard metal inserts having protruding portions extending from base portions that are secured in the cone cutter, or may comprise steel teeth that are milled, cast, or otherwise integrally formed from the cone material. In either case, the present ridge-cutting cutter elements are positioned on the cutter body in the areas between primary cutter elements where ridges may tend to build up, or are positioned to provide support to the outer surface of the primary cutter elements. The ridge-cutting cutter element""s protruding portion can be any shape such as: conical, chisel, round, or flat. It is preferred that the cutting portion have cutting edges to aggressively cut the ridge. Also, an individual cutter element can be rotated about its longitudinal axis so as to provide a more effective cutting action. For example, a chisel insert that is used to cut a ridge can be rotated to have its elongated crest positioned circumferentially on the cone cutter.
Another benefit can be realized by placing the ridge-cutting cutter element adjacent to the primary cutter element. In this embodiment, the protruding portion of the ridge cutter element can have a flank or edge positioned to divert the drilling fluid away from the cone material that is supporting the primary cutter element. This prevents excessive erosion around the primary cutter element.